1. Field of the Invention
The present invention relates to an inductor, and more particularly, to a multi-level symmetrical inductor made with integrated circuit technology.
2. Description of the Prior Art
As semiconductor technology advances, to facilitate small low cost devices, wireless communication chips are frequently integrated with passive elements such as inductors, transformers, and capacitors. Inductors, for instance, are required by wireless integrated circuits for devices such as low noise amplifiers (LNA), mixers, voltage controlled oscillators (VCO), resistor matched networks, and filters.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a one-level spiral inductor 10 according to the prior art. A coil forms the planar inductor 10, which has two ends p1 and p2, and spirals around a point 0 from the outer end p1 to the inner end p2 forming a specific number of loops. Because the coil of the inductor 10 cannot directly overlap itself, the overlapping section of the coil in FIG. 1 is connected to another induction layer by a via plug connected to the end p2. A major drawback of the one-layer spiral inductor 10 according to the prior is that the inductor consumes a large chip area, increasing the cost and reducing the possibility of integrating it onto a chip. In addition, the quality factor of the inductor 10 and the resistance of the coil are inversely proportional, that is, the longer coil, the larger resistance. Thus, increased energy dissipation of the inductor 10 reduces the quality factor, and it become difficult to apply such an inductor in wireless integrated circuit design.
Please refer to FIG. 2. FIG. 2 is a schematic diagram of two-level spiral inductor 12 according to the prior art. For saving chip area, as shown in FIG. 2, the inductor 12 is designed as two layered coils. The inductor 12 has two ends p1 and p2 and spirally circles a point C starting at the outer end p1 from an outer ring to an inner ring for a desired number of loops, which is then connected to another layer by a via plug, and spirals from an inner ring to an outer ring finally connecting to the end p2. It deserves to be mentioned that current flowing in the two layered coils is in the same direction which increases the mutual inductance of the inductor 12, that is, the current flows into the end p1 from the outer ring to the inner ring in a clockwise direction then connects to the second layer by the via plug, and similarly flows clockwise from the inner ring to the outer ring to the end p2.
The two-layer inductor 12 reduces chip area and increases mutual inductance between the two layers. In the same area of chip, the inductance of the two-level inductor 12 is 2–4 times higher than that of the one-layer inductor 10. In other words, for the same inductance, the two layer inductor 12 needs only ½–¼ of chip area the one-layer inductor 10 requires. In addition, the two-layer inductor 12 needs fewer coils for the same inductance as the one-layer inductor 10. As fewer coils provide a lower resistance, the two-layer inductor 12 has a higher quality factor. Though the two-layer inductor 12 has a smaller area and a better quality factor, more and more wireless communication chip designs use differential circuits to reduce common mode noise, and inductors applied such a differential circuit should be symmetrical to prevent common mode noise. This symmetry means that the inductor has the same structure as seen from any end. In FIG. 2, the end p1 and the end p2 of the inductor 12 are not symmetrical, and if the inductor 12 is applied in a differential circuit, it will not suitably prevent common mode noise.
From the above mentioned, the one-layer spiral inductor 10 according to the prior art needs larger chip area, which means increasing cost. Additionally, as longer coils also have larger resistances, much energy is lost and the quality factor is poor. The two-level inductor 12 improves the area and the quality factor, however, is not symmetrical so that if the inductor 12 is applied in a differential circuit, it cannot prevent common mode noise.